With the rapid increase in the amount of communication services, licensed spectrum of 3GPP (the 3rd Generation Partnership Project) is increasingly inadequate to provide higher network capacity. In order to further improve the utilization of spectrum resources, 3GPP is discussing how to use unlicensed spectrum, such as 2.4 GHz and 5 GHz frequency band, with the help of the licensed spectrum. Currently, the licensed spectrum is mainly used in Wi-Fi, Bluetooth, Radar, and medical and other systems.
Normally, access technology such as LTE designed for the licensed spectrum, which has high requirements on spectrum efficiency and user experience optimization, is not suitable for use in unlicensed frequency band. However, Carrier Aggregation (“CA” for short) function has made it possible to deploy LTE in the unlicensed spectrum. In addition, 3GPP has proposed LTE Assisted Access (“LAA” for short) concept, which can make use of the unlicensed spectrum with the aid of the licensed spectrum of LTE. For the unlicensed spectrum, there can be two working modes: 1) Supplemental Downlink (“SDL” for short) mode in which only downlink transmission sub-frame is involved; this mode can only be used with the aid of CA technology; 2) TDD mode in which both downlink sub-frame and uplink sub-frame are involved; this mode can be used with the aid of CA technology, Dual Connectivity (“DC” for short) technology, or can be used separately.
Compared with Wi-Fi system, LTE system operating in unlicensed frequency bands can provide higher spectral efficiency and greater coverage effect, and at the same time make data flow switch seamlessly between the licensed and unlicensed bands based on the same core network. For users, this means better broadband experience, a higher rate, and better stability and mobile convenience.
Access technology such as Wi-Fi for unlicensed spectrum has weak anti-interference ability. Wi-Fi system has designed a plurality of interference avoidance rules so as to avoid such interference, among which there is a Carrier Sense Multiple Access/Collision Detection (“CSMA/CD” for short) method. The basic principle of the CSMA/CD method is, before transmitting/receiving signaling or data, an Access Point (“AP” for short) or terminal will monitor and detect whether there is any other AP or terminal transmitting/receiving signaling or data around; if yes, the AP or terminal will keep on monitoring until no other AP or terminal is detected, otherwise, a random number will be generated as avoiding time. During the avoiding time, if no signaling or data transmission is detected, at the end of the backoff time, the AP or terminal will start to transmit signaling or data. The process is illustrated in FIG. 1.
In LTE network, since good orthogonality thereof can guaranty anti-interference performance, for downlink/uplink transmission between a base station and a user, there is no need to consider whether there is any other base station or user transmitting data. If the usage of other devices on the unlicensed frequency band is not considered when LTE is operating in the unlicensed frequency band, Wi-Fi equipment will be greatly disturbed. The reason is, since there is no monitoring rules in LTE, there will be transmission as long as there is business; thus, if there is LTE service transmission, Wi-Fi devices will need to wait for the LTE service transmission to complete and to detect an idle channel state for data transmission.
As can be seen, for LTE network, the most important key point when using unlicensed frequency band is to guaranty the coexistence of LAA and the existing access technology such as Wi-Fi. In traditional LTE system, there is no listen before talk (“LBT” for short) scheme to avoid collision.
Therefore, how to guaranty LTE system can operate normally in the unlicensed frequency band and at the same time avoid larger interference on other systems has become a problem to be solved.